This project is to elucidate the cellular target and mechanism of the antiproliferative natural products OSW-1, schweinfurthin A, ritterazine B, and cephalostatin 1 (so-called CRAMs). Since these small molecules have shown exciting activity against cultured human cancer cells lines, and in some cases, in vivo activity in mouse xenograft cancer models, uncovering their molecular mechanisms may reveal new protein targets for treating cancer. We have discovered that CRAMs bind OSBP (oxysterol binding protein) and perturb its activity in cells. Further evidence we have obtained suggests that ORPs (OSBP-related proteins) are likely mediating the cytotoxic activity of CRAMs. In this grant, we will 1) Determine which ORPs are mediating the cytotoxic activity of CRAMs and 2) Explore Why CRAMs are Cytotoxic to Cancer Cells. We anticipate that these studies will clarify the mode of action of CRAMs, demonstrate that OSBP/ORPs are unanticipated proteins required for cancer cell survival and reveal OSBP/ORPs as new protein targets for the treatment of cancer. In addition, we will disclose that CRAMs are the first small molecules that potently and specifically perturb OSBP/ORPs, enabling studies of their cellular functions. PUBLIC HEALTH RELEVANCE: This project will elucidate the cellular target and mechanisms of antiproliferative small molecules with application to cancer treatment. Knowledge gained in these studies may also reveal new functions of the OSBP/ORP family of proteins with possible applications in atherosclerosis and Alzheimer's disease.